1. Field of the Invention
The present invention generally relates to an improved filter sheet having its edges sealed with wax. More particularly the present invention relates to a filter sheet sealed along its edges with a non-microcrystalline wax. More specifically, an embodiment of the present invention relates to a filter sheet sealed along its edges with a wax having a durometer reading of about 75 to about 100 Shore A hardness at about 25.degree. C. and about 85.degree. C., a melt point of about 110.degree. C. to about 180.degree. C., and a viscosity of less than about 100 cP at about 155.degree. C. Further, an embodiment of the present invention relates to a filter sheet sealed along its edges with a wax comprising ethylene bis stearamide. Sealing of the edges of a conventional filter sheet with such waxes, and in particular a wax comprising ethylene bis stearamide, significantly increases the useful life of the filter sheet while providing a sheet which is easily released from a filter press and which retains integrity along the seal during repeated hot water sanitizations.
2. Background of the Related Art
Filtration is the process of separating particles from a fluid suspension (liquid or gas) by use of a porous medium or by means of a medium possessing chemical properties, such as hydrophobicity, electrostatic charge, etc., which permit such medium to interact and hold the particles which are to be separated from the fluid while permitting the fluid to pass there through. In conventional filtration, the filter medium retains most of the suspended particles which are filtered on or within itself, but allows the fluid being filtered to pass through unimpeded. Flow across the filter medium is generally achieved by the application of a driving force, usually in the form of a static pressure difference across the filter, which may be generated, for example by external pressure applied upstream, a vacuum applied downstream, gravity, or centrifugal force. Fluid suspension separations are used extensively in the manufacture of polymer products, medicinals, mineral and metallurgical processing, petroleum refining, water purification, emissions control, and in beverage and food preparation.
Most conventional filters may be categorized into two broad categories of filters: surface-type or cake filters, and depth-type filters. In surface-type filters solids form a filter cake on the surface of the filter medium, the cake itself forming a filter which clogs with time. Particles larger than the pore size of the filter medium are stopped at the upstream surface of the filter. In depth filtration, on the other hand, the filters are constructed of medium of sufficient thickness to filter along its bulk. Typically the filter medium has pore sizes much greater than the particles to be removed, so that the particles can penetrate into the medium. The particles are retained in the medium by means of random path low-efficiency particle captures and by adsorptive surface forces (molecular and electrostatic).
A well-known, and widely employed filter assembly, which may be described as a combination of a surface-type and depth-type filter, is the filter press. The basic filter press consists of either a series of recessed plates, or an alternating combination of plates, and cake-holding frames, a structure to support these elements, and a system for closing the elements together with enough force on the sealing faces to counterbalance the forces generated during filtration. Each plate is covered with a filter medium which acts as the filtration surface. Each plate and frame typically utilizes a unitary liquid feed conduit provided with openings at spaced intervals along its length and extending thorough the stacked plates as a means to introduce influent fluid into the flow channels defined between adjacent plates in the stacked plate assembly and through the filter medium and out an exit port within each plate. Typically, a unitary liquid withdrawal conduit featuring openings at spaced intervals along its length correspondingly extends through the staked plates and conveys the filtered fluid out of the filter press. As filtrate passes through the filter medium, a cake of solids is built up in the spaces between the plates on the filter medium on the "feed liquid sides" (i.e., activate filtration surfaces of the filter sheets). Conventionally, once the filter press is filled with solid filter cake material, the filter medium is either backwashed or filtration is stopped and the cake is washed from the press, or the press is opened, the plates separated and the solid filter cake removed. If filtration is stopped, the filter medium can be removed for washing or replacement and the system drained of liquid. Automation of the press may include opening and closing the press and shifting the plates in a manner to dump the cakes. Conventional filter presses commonly operate at pressures up to 100 lb./sq. in., and with special construction, metal presses may be operated up to 1000 lb/sq. in. The filter press has a number of advantages, including but not limited to,. simplicity, relatively low cost, flexibility and the ability to operate at high pressures as either a surface-type filter or a clarifying filter.
Filter media used on filter presses are referred to as filter sheets which include filter cloths and filter felts. The construct of such filter sheets optimally provides minimum flow resistance, offers resistance to blinding and ease of sanitation, and proffers strength and durability. Permeability and porosity are prime qualities to be considered in material selection and manufacture of the filter sheet. Filter sheets are frequently spun from monofil, multifil and/or staple fibers and filter felts, such as, for example, polyesters, viscose, polypropylene-polyamide, polyacrylonitrile, cotton, coconut fiber, sisal, hemp or any mixture of the preceding.
In order to permit easy replacement of spent filter sheets, filter sheets are conventionally held by physical means between the plates of a filter press rather than being integral and non-detachable therewith. A major problem with such detachable engagement between the plates is that any portion of the filter sheet perimeter extending over the plate tends to wick, by capillary action, filtrate from the filtration surface of the filter sheet to the outermost perimeter of another plate. The wicked fluid further often causes the filter sheet to stick to the plate at the point of its interface with the plate interfering with smooth opening of the press. Sticking, especially between plate surfaces designed to interface or articulate with adjoining plate surfaces, may further decrease the durability of multiple-use filter sheets.
To overcome the wicking problem filter sheets are known wherein the edges are coated with a latex material, such as polychloroprene. As noted in U.S. Pat. No. 4,212,746, the problem with latex materials, such as polychloroprene, is that, frequently, they do not sufficiently prevent capillary action of the fabric and exacerbate sticking between the plate and the edge-sealed filter sheet. This U.S. patent describes a filter sheet edge seal having one or more layers of a polychloroprene latex mixture containing zinc oxide and a silicate filler and a polychloroprene with carboxyl groups, which is said to greatly reduce sticking, thereby increasing durability. However, the manufacture of even the edge seals described in U.S. Pat. No. 4,212,746 pose problems.
For example, such latex seal must be appropriately cured prior to application of the filter sheet to the pressure plate. As liquid-tight latex seals frequently require application of multiple layers, multiple curing steps are frequently needed, involving considerable lost production time and capital expense. U.S. Pat. No. 4,212,746 discloses that it is particularly advantageous to dip the filter sheet into another chloroprene latex mixture of higher viscosity after the sheet has dried, the twice immersed filter sheets being said to be completely liquid-tight and gas-tight. Further, such seals, relying on multi-component compositions, suffer from the inherent increased danger of leaching of one or more of its components into the filtrate and/or reaction of the seal components with solubilized materials in the filtrate.
The employment of filter sheets in the filtration of suspensions containing organic materials has been particularly problematic. The life of such filters tends to be determined by microbial growth through the filter sheets rather than the durability of the sheet, or contaminant plugging. When such microbial infiltration into the filter sheet occurs, filtration must be suspended in order to avoid contamination of the product. Hot fluid sanitizing flushes of the filters are typically used to aid in reducing the growth of microbes. The use of anti-microbial agents on the filter sheet is typically not a practicable solution as often the product being filtered is a food stuff wherein consumption is predicated on exacting purity considerations.
There is a need, therefore, for better filter sheet edge seals which do not require intermediate curing in the manufacture process, which do not require multi-component compositions which increase the risk of contaminate leachate from the seal and reaction of the seal components with solubilized materials in the filtrate, and the composition of which simplify the production of edge sealed filter sheet.